For calls to gnutls-serv and gnutls-cli where --priority is not
specified, explicitly add the default value: --priority=normal. This is
needed for some tests on Ubuntu 20.04 (gnutls 3.6.13).
For example:
./ssl-opt.sh -f "DTLS fragmenting: gnutls.*1.0"
requires this PR to work on Ubuntu 20.04
Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>